Slovenia, Japan, and Sweden are also pretty surprising.
Don't know how you managed to take a train across the country and not notice this. Maybe you fell asleep?
If you look on a map, south korea looks to be densely covered with cities, one might think that a big part of the space between them would be agricultural areas.
Thinking of japan, I mostly remembered those vast urban areas around Tokyo, Osaka, Nagoya... and that much of the country is mountainous.
Maybe that's where I made the mistake to think about the japanese mountains like of the alps or even higher mountain ranges where there are no trees over the tree-line...
It have so many forest ok